A rotary machine such as a turbomachine (e.g., gas turbine, steam turbine, or other turbine system for power generation) is subjected to a variety of forces under operating conditions, particularly when accelerating or decelerating through critical speeds that excite a natural frequency of the system. The magnitudes of the forces and the resulting strains and vibrations on the machine components depend on the machine operating conditions and structural characteristics. Excessive vibration within the machine can result in excessive noise, excessive wear to internal seals and bearing surfaces, increase in secondary leakage in the turbine, and loss of turbine performance and efficiency.
The rotors that are employed in turbines are typically long and flexible, are often supported on tilting pads or fixed arc journal bearings, and often operate at speeds in excess of one or more critical speeds. During certain operating conditions, but more particularly during acceleration and deceleration, the rotor may operate at a critical speed (e.g., at a natural frequency) and may experience excessive vibration due either to mass imbalance in the rotor or flex of the rotor.